I resorted to an adjustable spanner to fit these, it can help to have the solenoid "live" when screwing it in, this holds the inner part up inside the body and prevents damage to the tip and it also prevents the thing falling into the hole, on Xantia TDs with the Bosch pump I have resored to a small chisel to avoid a lot of dismantling, my motorfactor carries stop solenoids carded for around £20
Stewart